There are various applications of LEDs in modern life, including traffic lights, rear lights of motorcycles, streetlights, computer indicators, and car lamps. In addition to necessary fabrication processes for LED chips, packaging processes for LEDs are also required.
The package form of the LED according to the prior art, owing to their appearance, is called bullet type for 5 mm type. This is the most mature packaging technology. Besides, this packaging method is also regarded the most economical and convenient by the packaging industry.
The packaging function for LED is to provide the necessary supports for LED chips in electricity, photonics, and heat dissipation. For example, when semiconductor devices are exposed to the atmosphere for a long time, they will age by moisture or other chemicals in the environment and resulting in degeneracy in property. An effective method for isolating the atmosphere is to use high-transparency epoxy to envelope LEDs. In addition, by selecting appropriate substrates, LEDs can be mechanically protected and thus improving their reliability substantially.
Moreover, LED packages still need excellent heat dissipation and optical extraction efficiency. In particular, the heat dissipation problem is more important. If the heat cannot be dissipation timely, the heat accumulated in devices will affect the lifetime and reliability of the devices.
Currently, the length of a side of the chip adopted by the LED according to the prior art is approximately 0.3 mm or smaller. Silver paste is used for attaching the chip on the reflector cup of the lead frame. The reflector cup is used for collecting the light emitted from the sidewall and reflecting the light to the desired direction. Next, a gold wire is used for connecting the positive terminal of the chip with a lead wire. The negative terminal is the reflector cup, which is connected with another lead wire. Then, high-temperature solidified epoxy is used for covering the top. While using the LED, a 20 mA current is used for driving. Around 90% of heat must be conducted from the negative terminal to the lead wire via the reflector cup and entering the PCB. Nonetheless, because the heat conductivity of the lead frame is inferior, having a high value of thermal resistivity of 250-300° C./W, the heat dissipation for the LED is inferior and hence leading to increase in temperature. The high temperature affects the packaging materials for the LED, including the epoxy. The epoxy starts yellowing, which, in turn, affects the light emitting efficient of the LED. In addition, color deviation also occurs and the lifetime of the LED.
Accordingly, the present invention improves the drawbacks of the prior art and provides an LED structure. The LED structure improves solves the yellowing problem in the packaging gel for LED chips as a result of the high temperature generated by the LED chips. Fluorescent powders are not packaged using the package gel. Instead, they are disposed on the LED chips.